Giratina's Altered Forme is a giant, dragon-like Pokémon, with an appearance similar to that of a large sauropod. It has 6 legs, gold protrusions that look like external ribs, and golden horns on its head. Its neck has five red stripes and 4 black stripes. On its back, it has a black stripe pattern that extends to its legs. It has black wings with three red thorn-like appendages called Girasols (a rare type of opal) coming out of them. Giratina's Origin Forme is more serpentine (like an amphitheater) than its Altered Forme. Its wings are now shaped like tentacles with the red thorn-like appendages sticking out at the end. Its mouth is now behind the two appendages in front of its face. They open when Giratina roars or etc. The horns on its head are still the same. Giratina in Origin Forme has five rows of golden spikes protruding out of the torso and near the tail. Five gray and red stripes are partially overlapped by these spikes.
Like some Pokémon, it has a signature move, called Shadow Force, which hits the opponent without fail even if the foe uses Protect or Detect beforehand. When in its Origin Forme, it retains its phenomenal amounts of HP, like any other Pokémon that is in their alternate formes, and it focuses on more offensive moves, and less on the defensive moves. It switches its Attack stats with its Defense stats in its Origin Forme, making it a great powerhouse. It is also a vastly powerful Pokémon that can use these benefits to fit the trainer's focus on attacking.
It will become Origin Forme when in the Distortion World, or when holding the Griseous Orb, which can be found in a secret part of the Distortion World that can only be entered through a portal in Turnback Cave (The portal will only open if you catch Giratina in the Distortion World) or get it before catching it.
Giratina, along with Dialga and Palkia, was created when Arceus hatched from it's egg. Arceus tasked Giratina to create anti-matter. Giratina was known (other than mewtwo) to be one of the most aggressive pokemon. Because of this, Arceus banished Giratina to the Distortion World, where he supposedly would be trapped in forever.
Giratina has a large role in the 11th Pokémon movie, Giratina and the Sky Warrior, where it tries to attack Dialga for polluting its dimension with its earlier battles with Palkia. However, later on it is captured by Zero and nearly killed after he tries to absorb its powers. Giratina is rescued by Ash and his friends, with help from Shaymin. Giratina also has a role in the 12th Pokémon movie, Arceus and the Jewel of Life, where Giratina fends off Arceus as Ash and friends prevent the betrayal long ago from happening.
Giratina has 6 neck spikes, 6 wing spikes, and 6 legs in its regular form; this is referencing 666, the Devil's Number.
Also, in the Bible, Satan is referenced to be a dragon, and Giratina also happens to be a Dragon-type.
Along with these resemblances, Giratina's species is the Renegade pokemon; this may also be referrencing the fact that in the Bible, Lucifer became "renegade", separating himself from God, before the creation of the world.
However, non of these speculations we're ever confirmed.
In the anime, Giratina has the same roar as the monster Mothra, who initially starred in her own movie, before being absorbed by the Godzillafranchise.
Giratina is not the only monster to have a roar from the Toho Sound Library. Palkia, Zekrom, Reshiram and Kyurem also have roars used by monsters in the Godzilla franchise (part of Palkia's roar came from Skywalker Sound).
If one listens closely, Girantina's cry sounds like it has background music.
Giratina is currently the only Pokemon to fall down from the top of the screen at the start of its wild encounter (Platinum only).
At the start of the Distortion World battle in Platinum, the message says "The Distortion World's Giratina appeared!". This is another one-off, but this only appears once. From there on, references state "The wild Giratina".